Cleanup names, bugfixes & add release notes

[devkral]

Changes:

• Factories have now different named variables
• Renamed is_proxy_model to __is_proxy_model__
• Rename meta.fields_mapping to meta.fields (otherwise names get too long (provide fallback))
• Internally all references are updated from Model.columns to Model.table.columns
• Internally all references are updated from Model.signals to Model.meta.signals
• Internally all references are updated from Model.fields to Model.meta.fields

Fixed:

• extract_db_fields uses now the assigned table if one is assigned.

Updated release notes.

[devkral]

shall we deprecate the Models.fields and Models.signals forwards?

[tarsil]

shall we deprecate the Models.fields and Models.signals forwards?

Not necessarily. Add a warning and let people know that in a couple of releases that will happen and they should be using the new one